We are currently investigating the control mechanisms regulating DNA replication in Escherichia coli and in mouse leukemia L5178Y cells. By means of temperature sensitive DNA initiation mutants of E. coli, we have been studying the events which lead to "premature" initiation, which occurs after release from conditions which block DNA synthesis and allows protein synthesis to continue. Our work indicates that when cells are returned to permissive temperature, after a period of 100 min at non-permissive temperature, low concentrations (25 - 30 mu g/ml of Chloramphenicol (CAP) allow what appears to be two rounds of residual synthesis, and high concentrations of CAP (150 - 200 mu g/ml) allow only one round. Rate experiments, however, indicate that low CAP may allow up to three and high CAP allow two initiation events. By means of DNA-cellulose chromatography and SDS gel electrophoresis we have identified a protein of about 60,000 - 65,000 mol. wt. which appears to be made immediately after the first initiation event upon release to permissive temperature. We have compared the nuclear envelope proteins of the L5178Y cells during log phases and stationary phase and during the various phases of the cell cycle. Results indicate that there are nuclear envelope proteins of about 54,000 and 57,000 mol. wt., as determined by SDS disc electrophoresis, that appear to be elevated during stationary phase. The various phases of the cell cycle show no significant differences in nuclear envelope proteins and the pattern is similar to that of log phase cells.